nfp: flower: Use struct_size() helper in kmalloc()

Make use of the struct_size() helper instead of an open-coded version,
in order to avoid any potential type mistakes or integer overflows that,
in the worst scenario, could lead to heap overflows.

Also, address the following sparse warnings:
drivers/net/ethernet/netronome/nfp/flower/tunnel_conf.c:359:25: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/174


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>